Method for producing potassium fluoroniobate crystals and potassium fluoroniobate crystals

ABSTRACT

Disclosed herein is a method for producing potassium fluoroniobate crystals by which highly pure, large-sized potassium fluoroniobate crystals can be obtained in high yield and which is advantageous from the viewpoints of material cost and material-dissolving operation; and potassium fluoroniobate crystals. This production method comprises the first and second steps (a) and (b) of (a) adding a potassium-containing electrolyte to a starting material comprising niobium to precipitate potassium oxyfluoroniobate and/or fluoroniobate as coarse crystals, and separating the coarse crystals by filtration, and (b) dissolving the coarse crystals in a recrystallization solvent that is an aqueous solution comprising 12 to 35% by weight of hydrofluoric acid and that has been heated to a temperature of 50° C. or more, and cooling the solution to 40° C. or lower at a cooling rate of less than 20° C./h to precipitate potassium fluoroniobate as crystals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing potassiumfluoroniobate crystals by which highly pure, large-sized potassiumfluoroniobate crystals can be obtained in high yield and which isadvantageous from the viewpoints of material cost andmaterial-dissolving operation, and to potassium fluoroniobate crystals.

2. Background Art

Potassium fluoroniobate is used as raw material in the production ofniobium powder. Since niobium powder has the effect of stabilizingcarbon present in steels to inhibit intergranular corrosion, it is usedas a steel additive, and this is the greatest use of niobium powder.Further, niobium alloys have been practically used for conductive tubesthat are placed in the light-emitting parts of high-pressure sodiumvapor lamps. Furthermore, niobium is employed as an additional elementin superconductive materials and superalloys. Niobium powder is nowbeing put to practical use for condensers.

A fractional crystallization process (Malinyak process) hasconventionally been known as a method for obtaining potassiumfluorotantalate crystals (K₂TaF₇) and potassium oxyfluoroniobatecrystals (K₂NbOF₅.H₂O) from a solution containing tantalum and niobium(see Zerikman, A. N., et al., “Niobu to Tantaru” (or Niobium andTantalum), pp. 61-64, Nisso Tsushin-sha, Tokyo, Japan). This process iscarried out in the following manner. A starting material, a solutioncontaining tantalum and niobium, is diluted to such an extent thatK₂NbOF₅ is not crystallized, and potassium chloride or the like is addedto the dilute solution to crystallize K₂TaF₇. The K₂TaF₇ crystals arefiltered off, and the filtrate is concentrated to precipitateK₂NbOF₅.H₂O as coarse crystals. The coarse crystals are then dissolvedin a 1-2 wt. % hydrofluoric acid solution to recrystallize potassiumoxyfluoroniobate (K₂NbOF₅.H₂O) from this solution. A problem with thisprocess is that it is not easy to remove, as K₂TaF₇ crystals, all of thetantalum from the starting material, so that the coarse K₂NbOF₅.H₂Ocrystals are to inevitably contain tantalum. It is difficult to reducethis tantalum remaining in the coarse K₂NbOF₅.H₂O crystals even byrecrystallization. This problem of tantalum contamination does not occurif a starting material containing niobium but no tantalum is used.

Another problem with the above process is that, in the above-describedstep of recrystallization, recrystallized from the hydrofluoric acidsolution having a low concentration of 1 to 2% by weight is potassiumoxyfluoroniobate (K₂NbOF₅.H₂O) rather than potassium fluoroniobate. Sucha problem never occurs when the corresponding tantalum-containing coarsecrystals are subjected to the similar recrystallization; this problem istherefore peculiar to niobium. To obtain potassium fluoroniobatecrystals, it is necessary to further conduct recrystallization by theuse of a hydrofluoric acid solution having a concentration of as high as12 to 15% by weight (see Zerikman, A. N., et al., “Niobu to Tantaru”, p.107, Nisso Tsushin-sha, Tokyo, Japan). The reason why potassiumfluoroniobate crystals are needed is that, since potassiumoxyfluoroniobate contains a large amount of oxygen, it is not adequateas a starting material for the production of niobium powder.

The above-cited reference “Niobu to Tantaru” also describes thefollowing: potassium fluoroniobate (K₂NbF₇) is more stable thanpotassium oxyfluoroniobate (K₂NbOF₅.H₂O) in a hydrofluoric acid solutionhaving a high concentration of 7% by weight or more. If this descriptionis taken into consideration, it might be possible to produce, withouteffecting recrystallization, potassium fluoroniobate crystals in onestep by adding a potassium-containing electrolyte to a niobium solutioncontaining hydrofluoric acid in high concentration. However, we nowfound that, even if this method is adopted, potassium oxyfluoroniobatesof other types, identified as K₃Nb₂F₁₁O, for instance, by X-raydiffractometry are inevitably present in the resulting crystals unlessthe concentration of hydrofluoric acid in the niobium solution exceeds30%. It is therefore not so easy to obtain satisfactorily highly purepotassium fluoroniobate even by this method.

Thus, crystals synthesized from a niobium solution tend to contain, inaddition to potassium fluoroniobate (i.e., K₂NbF₇), a small or largeamount of one or more potassium oxyfluoroniobates (i.e., K₃Nb₂F₁₁O,KNb₂O₅F, KNbO₂F and/or K₂NbOF₅.H₂O etc.) regardless of whether theconcentration of hydrofluoric acid in the niobium solution is high orlow. Moreover, it is often observed that only potassium oxyfluoroniobatecrystals precipitate.

SUMMARY OF THE INVENTION

We now found the following: if the first step of forming coarse crystalsthat may contain potassium oxyfluoroniobate and the second step ofsubjecting the coarse crystals to recrystallization using a hydrofluoricacid solution whose concentration is at least 12% by weight are effectedin combination, highly pure potassium fluoroniobate crystals can beobtained, without forming unwanted by-products, by using as a startingmaterial a niobium solution obtainable from solvent extraction, which isinexpensive. Namely, we found that it is possible to produce potassiumfluoroniobate crystals in high yield by a method advantageous from theviewpoints of material cost and material-dissolving operation. We alsofound that it is possible to obtain sufficiently large-sized potassiumfluoroniobate crystals in a large amount per operation if thetemperature and cooling rate in the recrystallization step are properlycontrolled.

An object of the present invention is therefore to provide a method forproducing potassium fluoroniobate crystals by which highly pure,large-sized potassium fluoroniobate crystals can be obtained and whichis advantageous from the viewpoints of material cost andmaterial-dissolving operation; and potassium fluoroniobate crystals.

To fulfil the above object, the present invention provides a method forproducing potassium fluoroniobate crystals, comprising the first andsecond steps (a) and (b) of:

(a) adding a potassium-containing electrolyte to a starting materialcomprising niobium to precipitate potassium oxyfluoroniobate and/orfluoroniobate as coarse crystals, and separating the coarse crystals byfiltration; and

(b) dissolving the coarse crystals in a recrystallization solvent thatis an aqueous solution comprising 12 to 35% by weight of hydrofluoricacid and that has been heated to a temperature of 50° C. or more, andcooling the solution to 40° C. or lower at a cooling rate of less than20° C./h to precipitate potassium fluoroniobate as crystals.

Further, the present invention provides potassium fluoroniobate crystalsconsisting essentially of potassium fluoroniobate, containing 30% byweight or more of crystals having sizes of 0.5 mm or more as determinedby sieve analysis.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a flow chart showing one example of the process of potassiumfluoroniobate formation in the production method according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The method for producing potassium fluoroniobate crystals according tothe present invention and potassium fluoroniobate crystals of theinvention will be specifically described hereinafter.

Method for Producing Potassium Fluoroniobate Crystals

The process of producing potassium fluoroniobate crystals according tothe present invention is shown in FIG. 1. As shown in this figure,potassium fluoroniobate crystals are produced via a first step (a) inwhich coarse potassium oxyfluoroniobate and/or fluoroniobate crystalsare formed by synthesis and a second step (b) in which the coarsecrystals are subjected to recrystallization.

Starting Material

In the production method according to the present invention, a solutioncomprising niobium is used as the starting material. To prepare theniobium solution, any one of various known methods can be adopted, and,moreover, any solvent can be used. For example, a purified aqueousniobium solution obtained from the process of solvent extraction oftantalum/niobium, which is widely effected in the industrial fields, canbe used as the starting material. It is also possible to use an aqueoussolution that is obtained by filtering a solution prepared bydissolving, in hydrofluoric acid, niobium-containing scraps or the likecontaining only a small amount of impurities, and that has not beensubjected to solvent extraction.

There is no particular limitation on the concentration of niobium in thestarting material. It is however preferable to make the concentration ofniobium in the starting material from 60 to 400 g/l, more preferablyfrom 100 to 300 g/l. As long as the concentration of niobium is in thisrange, crystals precipitate in a large amount per operation, and highyield can thus be attained. In addition, the stirring and filtration ofthe fluid containing the crystals that have precipitated can be effectedwithout difficulty. It is enough that the purity of the niobium in theniobium solution be at least 99% (e.g., approximately 99.9% to 99.99%).It is not necessary to make the purity higher, but, of course, niobiumhaving higher purity may be used. It is noted that the amount oftantalum, one of impurities, cannot be decreased even by the productionmethod according to the present invention. Therefore, to producepotassium fluoroniobate crystals having extremely high purity, it isdesirable to use a starting material comprising a niobium solution fromwhich tantalum has fully been removed by solvent extraction or the like.

According to a preferred embodiment of the present invention, theniobium solution comprises water and/or hydrofluoric acid. This meansthat water may be added for dilution when the concentration of niobiumin the solution is excessively high, and that hydrofluoric acid may beincorporated as the source of hydrofluoric acid to be used in thesynthesis of potassium fluoroniobate crystals. It is noted thatpotassium fluoride, which may be used as a potassium-containingelectrolyte that will be described later, also serves as the source ofhydrofluoric acid. Therefore, it is not always necessary to incorporatehydrofluoric acid into the niobium solution if potassium fluoride isadded as the potassium-containing electrolyte.

In the case where hydrofluoric acid is incorporated into the startingmaterial, it is preferable to make the concentration of hydrofluoricacid in the starting material 35% by weight or less, more preferably 30%by weight or less, most preferably 25% by weight or less. AS long as theconcentration of hydrofluoric acid in the starting material is in thisrange, the desired crystals can be obtained in satisfactorily high yieldif the concentration of niobium in the starting material and the amountof the potassium-containing electrolyte added are adequate. Moreover,the production facility hardly undergoes corrosion, and the startingmaterial can easily be handled; a starting material containinghydrofluoric acid in a concentration in the above-described range isthus advantageous from the viewpoints of facility and operation. In thestarting material containing hydrofluoric acid, niobium is believed toexist in the form of K₂NbOF₅ and/or H₂NbF₇.

(a) First Step (Step of Forming Coarse Crystals)

In the first step of the production method according to the presentinvention, a prescribed potassium-containing electrolyte is added to theabove-described starting material to precipitate potassiumoxyfluoroniobate and/or fluoroniobate as coarse crystals. As mentionedpreviously, niobium tends to form oxyfluoride salts (e.g., K₃Nb₂F₁₁O)rather than fluoride salts (e.g., K₂NbF₇), and this tendency is markedas compared with tantalum, which resembles niobium in its properties. Inthis first step, it does not matter if oxyfluoride salts are produced.

Any electrolyte can be used as the potassium-containing electrolyte foruse in the present invention as long as it can release potassium ions.Preferable examples of potassium-containing electrolytes useful hereininclude potassium chloride, potassium carbonate, potassium hydroxide,potassium fluoride, and combinations thereof. Of these, potassiumchloride is preferred because it is easy to handle and inexpensive.Further, the use of potassium fluoride is advantageous from theviewpoints of facility and operation. This is because, if potassiumfluoride is used as the electrolyte, it is possible to synthesizepotassium fluoroniobate crystals by using a minimum amount ofhydrofluoric acid, or even without using it.

When potassium chloride is added as the potassium-containingelectrolyte, it is believed that the following reactions take place tomainly yield coarse potassium oxyfluoroniobate (K₃Nb₂F₁₁O) crystalsalthough reactions that take place herein are not limited to thefollowing ones:

2H₂NbF₅O+HF+3KCl→K₃Nb₂F₁₁O+3HCl and/or H₂NbF₇+2KCl→K₂NbF₇+2HCl.

In the present invention, the potassium-containing electrolyte may beadded as it is, that is, in solid form, or after it is made into asolution. It is however preferable to add the electrolyte in solid formbecause, by doing so, it is possible to minimize the amount of fluid inthe system and, consequently, that of waste water. It is preferable toadd the potassium-containing electrolyte in such an amount that themolar ratio of potassium to niobium will be from 2 to 10, morepreferably from 2 to 7. When the potassium-containing electrolyte isadded in an amount smaller than this range, potassium fluoroniobate isnot fully crystallized, so that the yield of potassium fluoroniobatecrystals becomes low. On the other hand, when the potassium-containingelectrolyte is used in an amount greater than the above-described range,the excess potassium-containing electrolyte that does not contribute tothe formation of crystals is to exist in a large amount; this isunfavorable from the viewpoint of cost.

According to a preferred embodiment of the present invention, thetemperature of the starting material at the time when thepotassium-containing electrolyte is added thereto is between 30° C. and70° C., and that of the starting material at the time when the coarsecrystals are filtered off is less than 30° C. By so controlling thetemperature of the starting material, it is possible to make thedifference between the solubility at the time of the addition of theelectrolyte and that at the time of filtration great; coarse crystalsthus precipitate in an increased amount. The reason why the temperatureof the starting material at the time when the potassium-containingelectrolyte is added thereto is made between 30° C. and 70° C. is asdescribed below. If this temperature is lower than 30° C., thedifference between this temperature and the temperature of the startingmaterial at the time of filtration is so small that crystals cannot growwell. The resulting crystals are therefore small in size, and it takes alot of time to separate such small-sized crystals by filtration. On theother hand, the heating cost is increased if the temperature of thestarting material is raised to higher than 70° C. In addition,excessively large-sized crystals are produced if the electrolyte isadded to the starting material at a temperature of more than 70° C.; itwill take a lot of time to dissolve such large-sized crystals in arecrystallization solvent in the second step. Further, the reason whythe temperature of the starting material at the time when the coarsecrystals are filtered off is made lower than 30° C. is as follows: aslong as this temperature is lower than 30° C., only a small amount ofniobium remains in the filtrate, so that the loss of niobium becomesalmost nil. The above two operations may also be effected at roomtemperature. To effect the operations at room temperature isadvantageous in that the operations require neither heating facility norcooling facility, although the temperature of the starting material atthe time when the potassium-containing electrolyte is added thereto maybecome lower than 30° C., and a little bit longer time may be requiredfor filtration, or although the temperature of the starting material atthe time when the coarse crystals are filtered off may become higherthan 30° C., and the yield of niobium may be somewhat lowered.

In the first step in the present invention, the fluid containing thecoarse crystals that have precipitated is filtered to separate thecoarse crystals. As mentioned previously, the coarse crystals thusobtained contain a small or large amount of potassium oxyfluoroniobatein addition to potassium fluoroniobate, or consist essentially ofpotassium oxyfluoroniobate.

According to a preferred embodiment of the present invention, the molarratio K/Nb in the coarse crystals (i.e., coarse potassiumoxyfluoroniobate crystals, or a mixture of coarse potassiumoxyfluoroniobate crystals and coarse potassium fluoroniobate crystals)is from 1.0 to 5.0. As long as this molar ratio falls in the aboverange, it is avoided in the subsequent recrystallization step thatcrystals precipitate in a decreased amount because of the lack of K. Itis also avoided that the solubility is lowered due to excess K to makecrystals precipitate in a decreased amount. Thus, a satisfactorily largeamount of crystals can be obtained by recrystallization.

In the production method according to the present invention, the coarsecrystals obtained are subjected to the below-described second step (b)of recrystallization.

(b) Second Step (Step of Forming Crystals by Recrystallization)

In the second step of the production method according to the presentinvention, a recrystallization solvent, which is an aqueous solutioncomprising 12 to 35% by weight of hydrofluoric acid, is heated to 50° C.or more; the aforementioned coarse crystals are dissolved in thisrecrystallization solvent; and the solution is cooled to 40° C. or lowerat a cooling rate of less than 20° C./h to precipitate potassiumfluoroniobate as crystals.

The recrystallization solvent for use in the present invention is anaqueous solution comprising hydrofluoric acid. The concentration ofhydrofluoric acid in the recrystallization solvent is from 12 to 35% byweight, preferably from 16 to 30% by weight. As long as theconcentration of hydrofluoric acid in the recrystallization solvent isin this range, potassium fluoroniobate crystals containing no potassiumoxyfluoroniobate can easily be obtained. In addition, the productionfacility hardly undergoes corrosion, and the starting material caneasily be handled; the above-described concentration range is thusadvantages from the viewpoints of facility and operation.

Further, the recrystallization solvent for use in the present inventionmay comprise hydrochloric acid in addition to hydrofluoric acid. Whenthe recrystallization solvent containing both hydrofluoric acid andhydrochloric acid is used, the solubility of potassium fluoroniobatebecomes higher in the entire temperature range. In this case, thesolubility is greatly increased at a temperature at which the coarsecrystals are dissolved in the recrystallization solvent (thistemperature being relatively high), whereas it is only slightlyincreased at a temperature to which the solution obtained is cooled(this temperature being relatively low). For this reason, potassiumfluoroniobate crystals are obtained in a large amount as compared withthe case where the recrystallization solvent containing no hydrochloricacid is used. It is preferable to make the concentration of hydrochloricacid in the recrystallization solvent from 1 to 10% by weight, morepreferably from 2 to 6% by weight if the solubility and the cost ofpotassium fluoroniobate are taken into consideration.

In the second step in the present invention, the coarse crystalsobtained in the first step (a) are dissolved in the recrystallizationsolvent after heating the recrystallization solvent to a temperature of50° C. or more, preferably a temperature between 50° C. and 70° C., morepreferably a temperature between 60° C. and 70° C. If therecrystallization solvent has been heated only to a temperature of lessthan 50° C., crystals precipitate in a decreased amount while thesolution is cooled, so that the yield of the crystals is low. On theother hand, to heat the recrystallization solvent to an excessively hightemperature is unfavorable from the viewpoints of facility andoperational cost. It is preferable to dissolve the coarse crystals inthe recrystallization solvent while holding the recrystallizationsolvent at the temperature raised. However, the following manner mayalso be adopted: the recrystallization solvent is heated to atemperature that is slightly higher than the target temperature, and thecoarse crystals are dissolved in this solvent while cooling the solventin the air so that the temperature of the solvent can reach the targettemperature when the dissolution of the coarse crystals is completed.

The coarse crystals may be dissolved in the recrystallization solvent inany amount. It is however preferable to make the concentration of thecoarse crystals in the recrystallization solvent slightly lower than thesolubility of potassium fluoroniobate, which is determined by thetemperature of the solution of the coarse crystals in therecrystallization solvent and by the concentrations of hydrofluoric acidand hydrochloric acid (if present) in the recrystallization solvent. Ifthe coarse crystals are dissolved in the recrystallization solvent in anexcessively large amount, a large part of the crystals remainundissolved. This brings about the precipitation of fine-sized crystalsand the contamination of impure coarse potassium fluoroniobate crystals.On the other hand, if the coarse crystals are dissolved in therecrystallization solvent in an insufficient amount, crystalsprecipitate only in a small amount.

In the second step in the present invention, the solution obtained bydissolving the coarse potassium fluoroniobate crystals in therecrystallization solvent is cooled to precipitate potassiumfluoroniobate as crystals. The solution is cooled to a temperature of40° C. or less, preferably a temperature of 30° C. or less. If thesolution is cooled only to a temperature of more than 40° C., crystalsprecipitate only in an unsatisfactorily small amount to make the yieldlow.

It is herein necessary that the solution be cooled at a rate of lessthan 20° C./h, preferably at a rate of less than 10° C./h, morepreferably at a rate of less than 6° C./h. As long as the cooling rateis in this range, satisfactorily large-sized potassium fluoroniobatecrystals can be obtained. This is because the rate of crystal growth isbelieved to be greater than that of the formation of fine-sizedcrystals. It is not necessary to make the cooling rate almost constantfrom the beginning to the end of the cooling operation; the cooling ratemay be changed within the above-described range in the course of thecooling operation.

According to a preferred embodiment of the present invention, theproduction method may further comprise, before the step of cooling thesolution of the coarse potassium fluoroniobate crystals in therecrystallization solvent, the step of removing fine particles remainingin this solution. If this step is effected, the precipitation offine-sized crystals that is brought about by fine particles does notoccur, and crystals that are larger in size can be obtained. Any ofvarious known methods can be adopted to remove the fine particles, and amethod using a mesh filter, for example, is preferred because it caneasily be effected. In the step of removing the fine particles, thetemperature of the solution might become low to make crystalsprecipitate if the solubility is not sufficiently high. It is thereforedesirable that the coarse potassium fluoroniobate crystals be dissolvedin the recrystallization solvent in an amount slightly smaller than thatin the case where the step of removing fine particles is not effected,or at a temperature slightly higher than that in the case where the stepof removing fine particles is not effected.

The fluid containing the crystals that have precipitated is subjected tofiltration to separate it into the crystals and filtrate. The crystalsthus obtained are potassium fluoroniobate crystals, an object of thepresent invention.

(c) Optional Steps

According to a preferred embodiment of the present invention, thepotassium fluoroniobate crystals obtained in the aforementioned step (b)are washed with an aqueous solution of a potassium-containingelectrolyte, and then dried. This step is effective in removing thoseimpurities attached to the potassium fluoroniobate crystals to maketheir purity higher. In this step, water should not be used to wash thepotassium fluoroniobate crystals. This is because water can dissolve thesurfaces of the potassium fluoroniobate crystals, and potassiumoxyfluoroniobate crystals can unfavorably precipitate at the dissolvedparts of the surfaces. This problem never occurs when the crystals arewashed with an aqueous solution of a potassium-containing electrolyte,and highly pure potassium fluoroniobate crystals containingsubstantially no potassium oxyfluoroniobate can successfully beobtained. It is noted that, even if potassium fluorotantalate crystalsare washed with water, potassium oxyfluorotantalate crystals do notprecipitate; it can therefore be said that the above-described problemis peculiar to potassium fluoroniobate crystals.

Any potassium-containing electrolyte can be used herein as long as itcan release potassium ions and does not react with potassiumfluoroniobate. Preferable examples of potassium-containing electrolytesuseful herein include potassium chloride and potassium fluoride. Thereis no particular limitation on the concentration of thepotassium-containing electrolyte in its aqueous solution; however, thisconcentration is preferably 50 g/l or more, more preferably 75 g/l ormore, most preferably 100 g/l or more.

According to a preferred embodiment of the present invention, theproduction method may further comprise the step of passing the potassiumfluoroniobate crystals obtained in the aforementioned step (b) through asieve having an opening of 3.35 to 6.7 mm. Excessively large-sizedcrystals are removed by this step, and those potassium fluoroniobatecrystals that are moderately large and uniform in size can be obtained.By lightly crushing those crystals remaining on the sieve, and passingthe crushed crystals through the sieve again, it is possible to minimizethe loss of the crystals.

According to a preferred embodiment of the present invention, theproduction method may further comprise the step of recycling, as a partor whole of the recrystallization solvent in the step (b), thesupernatant liquid of the fluid containing the potassium fluoroniobatecrystals that have precipitated and/or the filtrate obtained from thefiltration of the fluid. According to another preferred embodiment ofthe present invention, the production method may further comprise thestep of recycling, as a part or whole of the starting material, thefiltrate obtained from the filtration carried out in the step (b). Inthese embodiments, precious niobium resources can fully be utilizedwithout any loss. Moreover, the filtrate contains hydrofluoric acid, sothat it is possible to make the amount of hydrofluoric acid to be addedconsiderably small as compared with the case where the filtrate is notrecycled.

Potassium Fluoroniobate Crystals

The potassium fluoroniobate crystals obtained by the above-describedproduction method according to the present invention consist essentiallyof potassium fluoroniobate, and are large in size. They contain 30% byweight or more, preferably 50% by weight or more, more preferably 60% byweight or more, and most preferably 80% by weight or more of crystalshaving sizes of 0.5 mm or more as determined by sieve analysis. As longas the potassium fluoroniobate crystals have large sizes as describedabove, they are not blown up in a heating oven in an apparatus forreduction with sodium, so that they do not stain the inside of theapparatus, or are not lost. Thus, the potassium fluoroniobate crystalsof the invention are improved in handling properties. Further, it isconsidered that the purity of potassium fluoroniobate crystals dependson their sizes, and, in general, larger crystals have higher purity.This is another advantage of large-sized crystals.

According to a preferred embodiment of the present invention, thepotassium fluoroniobate crystals contain not more than 10% by weight,preferably not more than 5% by weight, more preferably not more than 3%by weight, and most preferably not more than 1% by weight of crystalshaving sizes of less than 0.045 mm as determined by sieve analysis. Aslong as the content of such small-sized crystals in the potassiumfluoroniobate crystals falls in this range, it is easy to quantitativelysupply the crystals to the reduction oven, and, moreover, dust isscarcely raised.

According to a preferred embodiment of the present invention, thepotassium fluoroniobate crystals contain not more than 1% by weight,preferably not more than 0.5% by weight, more preferably not more than0.1% by weight, and most preferably substantially 0% by weight ofcrystals having sizes of 4 mm or more as determined by sieve analysis.As long as the content of such large-sized crystals in the potassiumfluoroniobate crystals falls in this range, the quantitative supply ofthe crystals to the reduction oven can easily be done, and, in addition,even the central parts of the crystals fully undergo reduction.

EXAMPLES

The present invention will now be explained more specifically byreferring to the following Examples. However, these examples are notintended to restrict the scope of the invention in any way.

Example 1

Synthesis of Coarse Crystals (First Step)

By properly changing the composition of the starting material and thesynthesis conditions, coarse potassium oxyfluoroniobate and/orfluoroniobate crystals were obtained (Synthesis Examples 1 to 25). Thecompositions of the starting materials and the synthesis conditions usedin Synthesis Examples 1 to 25 are shown in Table 1.

In each Synthesis Example, 1000 ml of a starting material was preparedby blending a niobium solution (concentration of niobium determined byICP emission spectroscopic analysis: 295 g/l, solvent: an extremely thinaqueous solution of hydrofluoric acid), a 55 wt. % hydrofluoric acidsolution and pure water in amounts as shown in Table 1. Potassiumchloride as a precipitant was added to and reacted with the startingmaterial that was at room temperature (23 to 27° C., abbreviated to “RT”in the table) or had been heated to 60° C. Thereafter, to fullyprecipitate potassium oxyfluoroniobate and/or fluoroniobate as coarsecrystals, the reaction solution was cooled to room temperature in thecase where the starting material had been heated to 60° C., or allowedto stand as it was in the case where the starting material had not beenheated. The coarse crystals were then separated by filtration.

The coarse crystals thus obtained were placed in a PTFE(polytetrafluoroethylene)-made container, and dried in a thermostaticdryer at 120° C. for approximately 15 hours. The X-ray diffractionpattern of the coarse crystals was obtained by the use of an X-raydiffractometer, and used to determine the chemical composition of thecoarse crystals. Further, the amount of niobium contained in thefiltrate was measured by IPC emission spectroscopic analysis, and theyield was calculated on the basis of the amount of niobium contained inthe starting material, using the following equation:

Yield (%)=100−[(the amount of niobium contained in the filtrate)/(theamount of niobium contained in the starting material)]×100

The results are shown in Table 1.

TABLE 1 Starting material Components of Starting (1000 mL) Syn- materialConcen- Synthesis Conditions Results thesis Nb Pure tration Tempera-Molar X-ray diffraction Example Solution 55% HF Water of HF Nb Contentture KCl ratio Yield pattern No. mL mL mL % g/L mol ° C. g mol K/Nb %K₃Nb₂F₁₁O K₂NbF₇ 1 370 240 390 14.6 109.2 1.175 RT 200 2.683 2.28 94.2⊚ + ? X 2 370 240 390 14.6 109.2 1.175 RT 250 3.353 2.85 96.1 ⊚ + ? X 3370 240 390 14.6 109.2 1.175 RT 300 4.024 3.42 96.6 ⊚ + ? X 4 370 240390 14.6 109.2 1.175 RT 350 4.695 4.00 97.8 ⊚ + ? X 5 370 240 390 14.6109.2 1.175 RT 500 6.707 5.71 99.0 ⊚ + ? X 6 370 240 390 14.6 109.21.175 60 250 3.353 2.85 97.7 ⊚ + ? X 7 270 240 490 14.6 79.7 0.857 RT185 2.482 2.90 94.8 ⊚ + ? X 8 500 240 260 14.6 147.5 1.588 RT 340 4.5612.87 97.3 ⊚ + ? X 9 370 330 300 19.8 109.2 1.175 RT 250 3.353 2.85 97.5⊚ ◯ 10 370 330 300 19.8 109.2 1.175 RT 300 4.024 3.42 98.3 ◯ ⊚ 11 370330 300 19.8 109.2 1.175 RT 350 4.695 4.00 98.7 Δ ⊚ 12 370 420 210 24.9109.2 1.175 RT 250 3.353 2.85 97.9 ◯ ⊚ 13 370 420 210 24.9 109.2 1.175RT 300 4.024 3.42 98.8 ◯ ⊚ 14 370 420 210 24.9 109.2 1.175 RT 350 4.6954.00 99.2 Δ ⊚ 15 370 520 110 30.4 109.2 1.175 RT 250 3.353 2.85 98.6 Δ ⊚16 370 70 560 4.3 109.2 1.175 RT 250 3.353 2.85 65.2 ? X 17 370 70 5604.3 109.2 1.175 RT 300 4.024 3.42 73.3 ? X 18 160 240 600 14.6 47.20.508 RT 110 1.475 2.90 66.2 ⊚ + ? X 19 160 240 600 14.6 47.2 0.508 RT130 1.744 3.43 68.4 ⊚ + ? X 20 160 420 420 24.9 47.2 0.508 RT 110 1.4752.90 72.3 ⊚ + ? X 21 370 240 390 14.6 109.2 1.175 RT 145 1.945 1.66 78.8⊚ + ? X 22 370 420 210 24.9 109.2 1.175 RT 145 1.945 1.66 80.3 ⊚ + ? Δ23 370 420 210 24.9 109.2 1.175 60 145 1.945 1.66 81.9 ⊚ + ? Δ 24 370420 210 24.9 109.2 1.175 RT 130 1.744 1.48 50.3 ⊚ + ? X 25 690 40 2702.5 203.6 2.191 RT 1200 16.096 7.35 99.0 ? X Criteria employed forevaluating the X-ray diffraction patterns are as follows: ⊚: the mainpeak is observed, ◯: peaks are clearly observed, Δ: peaks are slightlyobserved, X: no peak is observed, and ?:peaks that seem to becharacteristic of potassium oxyfluoroniobates other than K₃Nb₂F₁₁O areobserved.

The data in Table 1 show that the coarse crystals obtained in the firststep can contain either potassium oxyfluoroniobate or potassiumfluoroniobate, or both of them depending upon the composition of thestarting material used and the synthesis conditions employed. Further,when the results of Synthesis Examples 1 to 15 are compared with thoseof Synthesis Examples 16 to 25, it can be understood that the coarsecrystals can be obtained in a high yield of more than 90%, morespecifically more than 94% as long as the first step of the invention iseffected under the preferable conditions (i.e., the concentration ofniobium in the starting material: 60 to 200 g/l, the concentration ofhydrofluoric acid in the starting material: 10 to 35% by weight, and theamount of the potassium-containing electrolyte to be added, as indicatedby the molar ratio of potassium to niobium: 2 to 10). Furthermore, theresults of Synthesis Example 25 show that, even if the concentration ofhydrofluoric acid in the starting material is low, coarse crystals canbe obtained in high yield if the molar ratio K/Nb is made high. It isnoted that “molar ratio K/Nb” in Table 1 means not the molar ratio of Kto Nb in the coarse crystals but the molar ratio of K in thepotassium-containing electrolyte to Nb in the starting material. Evenwhen the syntheses are carried out under the conditions that are notwithin the above-described preferable ranges (Synthesis Examples 16 to25), yields of more than 50% were attained. Although these yields (withan exception of the yield in Synthesis Example 25) are lower than thosein Synthesis Examples in which the syntheses were carried out under thepreferable conditions, it is needless to say that Synthesis Examples 16to 25 are also included in the present invention.

Example 2

Synthesis I of Potassium Fluoroniobate Crystals via Recrystallization(Second Step)

Potassium fluoroniobate was recrystallized from a recrystallizationsolvent, where the concentration of hydrofluoric acid in therecrystallization solvent and the amount of coarse potassiumfluoroniobate crystals to be dissolved in the recrystallization solventwere changed as shown in Table 2 (Recrystallization Examples 1 to 7).

Coarse potassium fluoroniobate crystals to be subjected torecrystallization were firstly prepared by conducting synthesis underthe same conditions as in Synthesis Example 2 in Example 1, providedthat the operation scale was made greater. Recrystallization solventsfor use in Recrystallization Examples 1 to 7 were then respectivelyprepared by blending a 55 wt. % hydrofluoric acid solution and purewater in amounts as shown in Table 2. In each Recrystallization Example,the recrystallization solvent was heated to 60° C., and the above coarsecrystals (synthesis product) were dissolved in the solvent in an amountas shown in Table 2 while keeping the temperature of the solvent; thesolution obtained was cooled to room temperature at a cooling rate of 5°C./h to precipitate potassium fluoroniobate as crystals; the crystalswere filtered off and washed with an aqueous potassium chloride solutionhaving a concentration of 100 g/l, thereby obtaining potassiumfluoroniobate crystals of the present invention. After measuring the wetweight, the crystals were placed in a PTFE-made container, dried in athermostatic drier at 120° C. for approximately 15 hours, and weighed.Further, the dried crystals were subjected to X-ray diffractometry, andthe X-ray diffraction pattern obtained was used to determine thechemical composition of the crystals. The results are shown in Table 2.

TABLE 2 Amount of Synthesis Product Recrystallization Solvent DissolvedAmount of Crystals Concen- dry obtained by Recrystal- Pure tration of(calcu- Recrystallization X-ray diffraction lization 55% HF Water TotalHF wet lated wet dry pattern Example mL mL mL % g g g g K₂Nb₂F₁₁O K₃NbF₇1 1410 3590 5000 17.0 1200 1107 715 685 X ⊚ 2 1670 3330 5000 20.0 980904 521 496 X ⊚ 3 2110 2890 5000 25.0 810 747 423 397 X ⊚ 4 2050 19504000 30.0 1000 922 535 515 X ⊚ 5 400 4600 5000 5.0 2280 2103 1668 1204 ⊚X 6 820 4180 5000 10.1 1900 1752 1334 1088 ⊚ X 7 1240 3760 5000 15.01360 1254 846 716 ⊚ ◯ Criteria employed for evaluating the X-raydiffraction patterns are as follows: ⊚: the main peak is observed, ◯:peaks are clearly observed, Δ: peaks are slightly observed, and X: nopeak is observed.

The data shown in Table 2 demonstrate the following. The crystalsobtained in Recrystallization Examples 1 to 4 where the concentration ofhydrofluoric acid in each recrystallization solvent used was 16% byweight or more, which was in the preferable range, were found to bepotassium fluoroniobate crystals of the present invention. On the otherhand, the crystals obtained in Recrystallization Examples 5 to 7 wherethe concentration of hydrofluoric acid in each recrystallization solventused was less than 16% by weight, which was not in the preferable range,were found to be crystals chiefly composed of potassium oxyfluoroniobatecrystals.

Further, coarse crystals were synthesized under the same conditions asin Synthesis Examples 7, 8, 11, 15, 16, 19 or 23 in Example 1, providedthat the production scale was made greater. These coarse crystals wererespectively subjected to recrystallization under the same conditions asin the above-described Recrystallization Example 2, and the crystalsthus obtained were subjected to X-ray diffractometry to obtain X-raydiffraction patterns. From the X-ray diffraction patterns, all of thesecrystals were found to have the chemical composition K₂NbF₇, and thusidentified as potassium fluoroniobate.

Example 3

Synthesis II of Potassium Fluoroniobate Crystals via Recrystallization(Second Step)

Coarse crystals different from those used in Example 2, having an X-raydiffraction pattern not having the main peak of K₃Nb₂F₁₁O or K₂NbF₇ buthaving peaks that seemed to be those of potassium oxyfluoroniobatesother than K₃Nb₂F₁₁O were subjected to recrystallization in the samemanner as in Example 2. Namely, the above coarse crystals were dissolvedin a recrystallization solvent, and potassium fluoroniobate wasrecrystallized from this solution, where the concentration ofhydrofluoric acid in the recrystallization solvent was changed as shownin Table 3 (Recrystallization Examples 8 to 14).

Coarse crystals to be subjected to recrystallization were firstlyprepared by conducting synthesis under the same conditions as inSynthesis Example 25 in Example 1, provided that the operation scale wasmade greater. As can be known from the data in Table 1, the coarsecrystals have such an X-ray diffraction pattern that, although the mainpeak of K₃Nb₂F₁₁O or K₂NbF₇ cannot be observed, peaks that seem to bethose of potassium oxyfluoroniobates other than K₃Nb₂F₁₁O are observed.It is noted that, since the ICDD card for X-diffraction analysis doesnot contain the chemical compositions K₂NbOF₅ and K₂NbOF₅.H₂O, it isdifficult to identify the “peaks that seem to be those of potassiumoxyfluoroniobates” as substances having these chemical compositions. Thecoarse crystals were subjected to recrystallization in the same manneras in Example 2, and then to X-ray diffractometry. From the X-raydiffraction pattern obtained, the chemical composition of the crystalswas determined. The results are shown in Table 3.

TABLE 3 Relationship between Concentration of Hydrofluoric Acid inRecrystallization Solvent and Chemical Composition of Crystal Recrystal-Concentration of HF lization in Recrystalli- X-ray Diffraction Examplezation Solvent pattern No. (wt. %) K₃Nb₂F₁₁O K₂NbF₇ 8 12.0 X ⊚ 9 15.0 X⊚ 10 17.0 X ⊚ 11 20.0 X ⊚ 12 25.0 X ⊚ 13 30.0 X ⊚ 14 10.1 ⊚ X Criteriaemployed for evaluating the X-ray diffraction patterns are as follows:⊚: the main peak is observed, ◯: peaks are clearly observed, Δ: peaksare slightly observed, and X: no peak is observed.

The data shown in Table 3 demonstrate the following. The crystalsobtained in Recrystallization Examples 8 to 13 where the concentrationof hydrofluoric acid in each recrystallization solvent used was 12% byweight or more were found to be potassium fluoroniobate crystals of thepresent invention. On the other hand, the crystals obtained inRecrystallization Example 14, in which the concentration of hydrofluoricacid in the recrystallization solvent used was less than 12% by weight,were found to be crystals consisting essentially of potassiumoxyfluoroniobate crystals.

Further, the results of Examples 1 to 3 show the following. Namely, itis believed that potassium oxyfluoroniobate crystals having the chemicalcomposition K₃Nb₂F₁₁O (the coarse crystals used in Example 2, preparedin accordance with Synthesis Example 2) can be prepared, in the firststep, under the conditions whose preferable range is wider than that ofthe conditions under which potassium oxyfluoroniobate crystals that donot have the chemical composition K₃Nb₂F₁₁O (the coarse crystals used inExample 3, prepared in accordance with Synthesis Example 25) can beprepared. However, as can be known from the results of Example 3, highlypure potassium fluoroniobate crystals (K₂NbF₇), an object of the presentinvention, can successfully be obtained even from potassiumoxyfluoroniobate crystals that do not have the chemical compositionK₃Nb₂F₁₁O. Moreover, in this case, desired potassium fluoroniobatecrystals can be obtained even by the use of a recrystallization solventwhose hydrofluoric acid content is as low as 12% by weight.

Example 4

Effects of Washing Conducted in Second Step

Potassium fluoroniobate crystals separated by filtration were washed inthe second step under different conditions to examine the effects ofwashing on the chemical composition of the crystals. The liquids usedfor this washing are shown in Table 4.

First of all, potassium fluoroniobate crystals were produced under thesame conditions as in Recrystallization Example 2 in Example 2, providedthat the recrystallization solvent and the coarse crystals were used inamounts 4 times larger than those in Recrystallization Example 2 andthat the crystals obtained by recrystallization were not dried.Thereafter, 200 g of the crystals obtained (wet) were washed with 100 mlof one of the liquids shown in Table 3, placed in a PTFE-made container,and dried in a thermostatic drier at 120° C. for approximately 15 hours.The dried crystals were subjected to X-ray diffractometry, and the X-raydiffraction pattern obtained was used to determine the chemicalcomposition of the crystals. The results are shown in Table 4.

TABLE 4 X-ray Diffraction Pattern Liquid Used for Washing K₃Nb₂F₁₁OK₂NbF₇ Not used (not washed) X ⊚ Pure water ◯ ⊚  10 g/l KCl Δ ⊚  50 g/lKCl X ⊚ 100 g/l KCl X ⊚ 250 g/l KCl X ⊚ Criteria employed for evaluatingthe X-ray diffraction patterns are as follows: ⊚: the main peak isobserved, ◯: peaks are clearly observed, Δ: peaks are slightly observed,and X: no peak is observed.

The results shown in Table 4 demonstrate that potassium oxyfluoroniobateis secondarily produced if the potassium fluoroniobate crystals arewashed with pure water, whereas potassium oxyfluoroniobate is notsecondarily produced when the crystals are washed with an aqueouspotassium chloride solution (preferably having a concentration of 50 g/lor more) Also in the case where the crystals are not washed at all,potassium oxyfluoroniobate is not secondarily produced. In this case,however, the crystals generate hydrogen fluoride vapor during the dryingprocess, so that the drier used can be damaged by this vapor. It is thusfound to be favorable that the potassium fluoroniobate crystals bewashed with an aqueous KCl solution (preferably having a concentrationof 50 g/l or more) after they are separated by filtration.

Example 5

Effects of Cooling Rate in Second Step

The effects of the cooling rate in the second step on crystal size wereexamined by changing as shown in Table 5 the cooling rate uponrecrystallization.

Specifically, potassium fluoroniobate crystals were produced under thesame conditions as in Recrystallization Example 2 in Example 2, providedthat one of the cooling rates shown in Table 5 was used. The crystalsize distributions of the crystals obtained by using different coolingrates were then respectively determined by sieve analysis. The sieveanalysis was conducted in the following manner. The potassiumfluoroniobate crystals obtained were firstly sifted through a sievehaving an opening of 4.00 mm, and the crystals remaining on this sievewere weighed. Those crystals that had passed through the above sievewere sifted through a sieve having an opening of 1.70 mm, and thecrystals remaining on this sieve were weighed. In this manner, thecrystals were then successively sifted through a sieve having an openingof 0.35 mm and that having an opening of 0.045 mm, and the crystalspassed through these sieves were respectively weighed. Percentages byweight of the crystals having sizes of less than 0.045 mm, those havingsizes of 0.045 mm or more and less than 0.35 mm, those having sizes of0.35 mm or more and less than 0.50 mm, those having sizes of 0.50 mm ormore and less than 1.70 mm, those having sizes of 1.70 mm or more andless than 4.00 mm, and those having sizes of 4.00 mm or more wereobtained by calculation. The results are shown in Table 5.

TABLE 5 Cooling Rate vs. Crystal Size Distribution (wt. %) Cooling Rate(° C./h) Range of Crystal Size (mm) 5 15 30 less than 0.045 0.6 1.5 18.00.045 or more and less than 0.35 6.0 14.5 43.0 0.35 or more and lessthan 0.50 11.3 26.5 25.0 0.50 or more and less than 1.70 40.3 50.0 13.01.70 or more and less than 4.00 41.8 7.5 1.0 4.00 or more 0.0 0.0 0.0total of 0.50 or more 82.1 57.5 14.0

The data in Table 5 show the following. In the case where the coolingrate was below 20° C./h (specifically, 5° C./h and 15° C./h), thecrystals obtained were found to contain more than 50% by weight (morethan 80% by weight when the cooling rate was 5° C./h,) of crystalshaving particle sizes of 0.50 mm or more; this means that large-sizedcrystals were obtained in a large amount. On the other hand, when thecooling rate was in excess of 20° C./h (specifically, 30° C./h), thecrystals obtained were found to contain only 10 percent level by weightof crystals having particle sizes of 0.50 mm or more; this means thatlarge-sized crystals were obtained only in a small amount.

Example 6

Chemical Composition Analysis

Some of the coarse crystals synthesized and the crystals finallyobtained via recrystallization (potassium fluoroniobate crystals) in theaforementioned Examples 1 to 5 were analyzed to determine the chemicalcompositions of their main components. Specifically, to determine thepercentages of K and Nb, the crystals were dissolved in hydrofluoricacid and aqua regia, respectively, and the solutions were subjected toICP emission spectroscopic analysis; and to determine the percentage ofF, the crystals were subjected firstly to alkali fusion, then toextraction with hot water, and finally to the fluoride ion electrodemethod. The results are shown in Table 6.

TABLE 6 K Nb F Molar Ratio (% by weight) K/Nb Synthesis Example 2 24.135.2 40.0 1.63 (Example of the Invention) Synthesis Example 25 34.6 23.928.0 3.44 (Example of the Invention) Recrystallization Example 2 25.430.5 44.0 1.98 (Example of the Invention) Recrystallization Example 325.6 30.8 43.5 1.98 (Example of the Invention) Recrystallization Example6 24.4 34.1 40.5 1.70 (Comparative Example) Recrystallization Example 724.8 34.2 40.8 1.72 (Comparative Example) Potassium Oxyfluoroniobate22.2 35.2 39.6 1.50 K₃Nb₂F₁₁O (calculated) Potassium Fluoroniobate 25.730.6 43.7 2.00 K₂NbF_(7 (calculated))

The data shown in Table 6 demonstrate the following. The coarse crystalsobtained in Synthesis Example 2 have the chemical composition nearlyequal to that of potassium oxyfluoroniobate. The crystals obtained inRecrystallization Examples 2 and 3 have the chemical compositions thatare almost identical to that of potassium fluoroniobate. The crystalsobtained in Recrystallization Examples 6 and 7 have the chemicalcompositions close to that of potassium oxyfluoroniobate. It is thusconfirmed that these results support the chemical compositions of thecrystals determined from the X-ray diffraction patterns in the aboveExamples.

Example 7

Synthesis III of Potassium Fluoroniobate Crystals via RecrystallizationUsing Recrystallization Solvent further Containing Hydrochloric Acid(Second Step)

Potassium fluoroniobate was recrystallized from a recrystallizationsolvent containing both hydrofluoric acid and hydrochloric acid, wherethe concentrations of hydrofluoric acid and hydrochloric acid in therecrystallization solvent and the amount of coarse potassiumfluoroniobate crystals to be dissolved in the recrystallization solventwere changed as shown in Table 7 (Recrystallization Examples 15 to 20).

Coarse potassium fluoroniobate crystals to be subjected torecrystallization were firstly prepared by conducting synthesis underthe same conditions as in Synthesis Example 2 in Example 1, providedthat the operation scale was made greater. Recrystallization solventsfor use in Recrystallization Examples 15 to 20 were then respectivelyprepared by blending hydrofluoric acid, hydrochloric acid and pure waterso that the concentrations of hydrofluoric acid and hydrochloric acid inthe solvent would be as shown in Table 7. In each RecrystallizationExample, the recrystallization solvent was heated to 60° C., and theabove coarse crystals (synthesis product) were dissolved in the solventin an amount as shown in Table 7 while keeping the temperature of thesolvent; the solution obtained was cooled to room temperature at acooling rate of 5° C./h to precipitate potassium fluoroniobate ascrystals; the crystals were filtered off and washed with an aqueouspotassium chloride solution having a concentration of 100 g/l, therebyobtaining potassium fluoroniobate crystals of the present invention.After measuring the wet weight, the crystals were placed in a PTFE-madecontainer, dried in a thermostatic drier at 120° C. for approximately 15hours, and weighed. Further, the crystals were subjected to X-raydiffractometry, and the X-ray diffraction pattern obtained was used todetermine the chemical composition of the crystals. The results areshown in Table 7.

TABLE 7 Amount of Amount of Recrystallization Solvent Synthesis CrystalsAmount Concen- Concen- Product obtained by Recrystal- of tractiontraction Dissolved Recrystallization X-ray diffraction lization Solventof HF of HCl wet dry wet dry pattern Example mL % % g g g g K₂Nb₂F₁₁OK₂NbF₇ 15 5000 17 2.5 1290 1190 798 755 X ⊚ 16 5000 17 5.0 1380 1273 866826 X ⊚ 17 5000 17 7.5 1460 1347 927 882 X ⊚ 18 5000 20 5.0 1130 1042635 608 X ⊚ 19 5000 25 5.0 940 867 515 492 X ⊚ 20 4000 30 5.0 1150 1061665 630 X ⊚ Criteria employed for evaluating the X-ray diffractionpatterns are as follows: ⊚: the main peak is observed, ◯: peaks areclearly observed, Δ: peaks are slightly observed, and X: no peak isobserved.

As shown in Table 7, the recrystallization solvents used inRecrystallization Examples 15 to 20 in this Example 7 containedhydrofluoric acid in concentrations similar to those of hydrofluoricacid in the recrystallization solvents used in RecrystallizationExamples 1 to 4 in Example 2, and further contained hydrochloric acid.Specifically, when viewed from the composition of the recrystallizationsolvent employed and the amount of the same used, RecrystallizationExamples 15 to 17 in Example 7 agree with Recrystallization Example 1 inExample 2; Recrystallization Example 18 in Example 7 agrees withRecrystallization Example 2 in Example 2; Recrystallization Example 19in Example 7 agrees with Recrystallization Example 3 in Example 2; andRecrystallization Example 20 in Example 7 agrees with RecrystallizationExample 4 in Example 2, provided that the recrystallization solventsused in Example 7 further contain hydrochloric acid. From the comparisonbetween the results of Recrystallization Examples in Example 7 and thoseof the corresponding Recrystallization Examples in Example 2, thefollowing can be known. Namely, when the recrystallization solventscontaining not only hydrofluoric acid but also hydrochloric acid wereused, the coarse crystals showed higher solubility at a temperature of60° C., so that it was possible to dissolve the coarse crystals in suchrecrystallization solvents in increased amounts. In fact, in thepreviously mentioned Recrystallization Examples 1 to 4 in Example 2, itwas impossible to dissolve the coarse crystals in the recrystallizationsolvents (containing no hydrochloric acid) in the same amounts as inExample 7. Further, in Example 7, the coarse crystals were dissolved inthe recrystallization solvents in increased amounts, and, as a result,crystals were obtained in drastically increased amounts viarecrystallization. Moreover, the crystals thus obtained were confirmedto be highly pure potassium fluoroniobate crystals containing nopotassium oxyfluoroniobate crystals.

What is claimed is:
 1. A method for producing potassium fluoroniobatecrystals, comprising the first and second steps (a) and (b) of: (a)adding a potassium-containing electrolyte to a starting materialcomprising niobium to precipitate potassium oxyfluoroniobate and/orfluoroniobate as coarse crystals, and separating the coarse crystals byfiltration; and (b) dissolving the coarse crystals in arecrystallization solvent that is an aqueous solution comprising 12 to35% by weight of hydrofluoric acid and that has been heated to atemperature of 50° C. or more to form a solution, and cooling thesolution to 40° C. or lower at a cooling rate of less than 20° C./h toform a precipitate potassium fluoroniobate as crystals in a fluid. 2.The method for producing potassium fluoroniobate crystals according toclaim 1, wherein the recrystallization solvent further contains 1 to 10%by weight of hydrochloric acid.
 3. The method for producing potassiumfluoroniobate crystals according to claim 1, wherein the coarsepotassium oxyfluoroniobate crystals comprise a substance that isidentified as K₃Nb₂F₁₁O by X-ray diffractometry.
 4. The method forproducing potassium fluoroniobate crystals according to claim 1, whereinthe molar ratio K/Nb in the coarse crystals is from 1.0 to 5.0.
 5. Themethod for producing potassium fluoroniobate crystals according to claim1, wherein the starting material comprises 60 to 400 g/l of niobium andnot more than 35% by weight of hydrofluoric acid.
 6. The method forproducing potassium fluoroniobate crystals according to claim 1, whereinthe potassium-containing electrolyte is added to the starting materialin such an amount that the molar ratio of potassium to niobium will befrom 2 to
 10. 7. The method for producing potassium fluoroniobatecrystals according to claim 1, wherein the recrystallization solvent isan aqueous solution comprising 16 to 30% by weight of hydrofluoric acid.8. The method for producing potassium fluoroniobate crystals accordingto claim 1, further comprising the step of recycling, as a part or wholeof the recrystallization solvent in the second step (b), the supernatantliquid of the fluid containing the potassium fluoroniobate crystals thathave precipitated and/or the filtrate obtained from the filtration ofthe fluid.
 9. The method for producing potassium fluoroniobate crystalsaccording to claim 1, wherein the potassium fluoroniobate crystalsobtained in the second step (b) are washed with an aqueous solution of apotassium-containing electrolyte, and then dried.
 10. The method forproducing potassium fluoroniobate crystals according to claim 1, furthercomprising the step of passing the potassium fluoroniobate crystalsobtained in the second step (b) through a sieve having an opening of3.35 to 6.7 mm.
 11. Potassium fluoroniobate crystals consistingessentially of potassium fluoroniobate, containing 30% by weight or moreof crystals having sizes of 0.5 mm or more as measured by sieveanalysis.
 12. The potassium fluoroniobate crystals according to claim11, containing not more than 10% by weight of crystals having sizes ofless than 0.045 mm as measured by sieve analysis.
 13. The potassiumfluoroniobate crystals according to claim 11, containing not more than1% by weight of crystals having sizes of 4 mm or more as measured bysieve analysis.
 14. The potassium fluoroniobate crystals according toclaim 12, containing not more than 1% by weight of crystals having sizesof 4 mm or more as measured by sieve analysis.
 15. The method forproducing potassium fluoroniobate crystals according to claim 2, whereinthe coarse potassium oxyfluoroniobate crystals comprise a substance thatis identified as K₃Nb₂F₁O by X-ray diffractometry.
 16. The method forproducing potassium fluoroniobate crystals according to claim 2, whereinthe molar ratio K/Nb in the coarse crystals is from 1.0 to 5.0.
 17. Themethod for producing potassium fluoroniobate crystals according to claim2, wherein the starting material comprises 60 to 400 g/l of niobium andnot more than 35% by weight of hydrofluoric acid.
 18. The method forproducing potassium fluoroniobate crystals according to claim 2, whereinthe potassium-containing electrolyte is added to the starting materialin such an amount that the molar ratio of potassium to niobium will befrom 2to
 10. 19. The method for producing potassium fluoroniobatecrystals according to claim 2, wherein the recrystallization solvent isan aqueous solution comprising 16 to 30% by weight of hydrofluoric acid.20. The method for producing potassium fluoroniobate crystals accordingto claim 2, further comprising the step of recycling, as a part or wholeof the recrystallization solvent in the second step (b), the supernatantliquid of the fluid containing the potassium fluoroniobate crystals thathave precipitated and/or the filtrate obtained from the filtration ofthe fluid.